From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S938940AbcIXHac (ORCPT ); Sat, 24 Sep 2016 03:30:32 -0400 Received: from mail-pa0-f65.google.com ([209.85.220.65]:34697 "EHLO mail-pa0-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933908AbcIXHaa (ORCPT ); Sat, 24 Sep 2016 03:30:30 -0400 Date: Sat, 24 Sep 2016 03:30:26 -0400 From: Alexei Starovoitov To: Daniel Borkmann Cc: "Naveen N. Rao" , linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, netdev@vger.kernel.org, Michael Ellerman , Alexei Starovoitov , "David S. Miller" , Ananth N Mavinakayanahalli Subject: Re: [PATCH 2/3] bpf powerpc: implement support for tail calls Message-ID: <20160924073025.GB48071@ast-mbp.thefacebook.com> References: <40b65ab2bb3a48837ab047a70887de3ccd70c56b.1474661927.git.naveen.n.rao@linux.vnet.ibm.com> <57E5ADD2.7090308@iogearbox.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <57E5ADD2.7090308@iogearbox.net> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Sep 24, 2016 at 12:33:54AM +0200, Daniel Borkmann wrote: > On 09/23/2016 10:35 PM, Naveen N. Rao wrote: > >Tail calls allow JIT'ed eBPF programs to call into other JIT'ed eBPF > >programs. This can be achieved either by: > >(1) retaining the stack setup by the first eBPF program and having all > >subsequent eBPF programs re-using it, or, > >(2) by unwinding/tearing down the stack and having each eBPF program > >deal with its own stack as it sees fit. > > > >To ensure that this does not create loops, there is a limit to how many > >tail calls can be done (currently 32). This requires the JIT'ed code to > >maintain a count of the number of tail calls done so far. > > > >Approach (1) is simple, but requires every eBPF program to have (almost) > >the same prologue/epilogue, regardless of whether they need it. This is > >inefficient for small eBPF programs which may not sometimes need a > >prologue at all. As such, to minimize impact of tail call > >implementation, we use approach (2) here which needs each eBPF program > >in the chain to use its own prologue/epilogue. This is not ideal when > >many tail calls are involved and when all the eBPF programs in the chain > >have similar prologue/epilogue. However, the impact is restricted to > >programs that do tail calls. Individual eBPF programs are not affected. > > > >We maintain the tail call count in a fixed location on the stack and > >updated tail call count values are passed in through this. The very > >first eBPF program in a chain sets this up to 0 (the first 2 > >instructions). Subsequent tail calls skip the first two eBPF JIT > >instructions to maintain the count. For programs that don't do tail > >calls themselves, the first two instructions are NOPs. > > > >Signed-off-by: Naveen N. Rao > > Thanks for adding support, Naveen, that's really great! I think 2) seems > fine as well in this context as prologue size can vary quite a bit here, > and depending on program types likelihood of tail call usage as well (but > I wouldn't expect deep nesting). Thanks a lot! Great stuff. In this circumstances approach 2 makes sense to me as well.